1. Field of the Invention
This invention relates to a method and apparatus for growing single crystalline material.
2. Discussion of Prior Art
Apparatus for growing crystals by the known Czochralski technique is described for example in GB 1,494,342. A seed crystal is dipped into a melt of the crystal to be grown then rotated and slowly withdrawn. By suitable adjustment of the melt temperature and the rotation and withdrawal rates a desired shape of crystal is grown. Ideally a crystal increases smoothly from the seed diameter up to the desired diameter and then continues at a uniform diameter until tapered off at growth end. Uniform growth diameter has been achieved for the difficult to grow III-V materials such as GaAs with apparatus as described in GB 1,494,342 where the growing crystal is weighed by a load cell during growth.
In this the crystal weight or some function of weight is compared with an expected value and any error which occurs is used to correct the power supplied to the melt and/or the pull speed.
One weakness of the method may be found during the grow-out phase when the system attempts to control the increase in diameter from a narrow seed to a much larger final diameter. The signals are initially small and the control of cone angle is not precise. A simple increase in system gain during this phase may help but can easily lead to instability or roughness of the crystal shape leading to crystalline faults.
Smooth control of cone angle is important in the growth of many materials including GaAs single crystals in order to achieve low densities of dislocations in the crystal.
A further and more severe problem arises when certain semiconductors (e.g. GaAs) are grown at low pull rates especially combination with isothermal melts required for better quality crystals. It is possible for the crystal to grow out across the surface of the melt and simultaneously to be growing down under the surface. In spite of this real growth of the crystal the weight sensor may detect little or no increase in weight of the crystal. This lack of weight change is due to the near horizontal surface tension forces (which therefore have little effect on the load cell) and also the fact that the density of the solid in the `anomalous` materials is less than that of the liquid. This means that the solid below the melt surface is buoyant and subtracts from the measured weight.
Under these conditions control may be lost completely and the melt surface freezes, often breaking the seed and terminating the run. This can be an expensive failure in production runs where large and costly melts are involved.